1. Field of the Invention
The present invention concerns a method for analysis by neutron activation to measure the quantity of one or more elements sought in a flow of material in bulk, in which the flow of material is made to pass close to at least one source of neutrons capable of inducing the emission of gamma rays from said elements, specific gamma rays emitted by the elements are detected, measured and sorted according to their energy spectra, to obtain their respective quantities in the flow of material. The invention also concerns an installation for the application of this method.
1. Description of the Prior Art
A method of this type is known notably in the mining industry, for determining the metal content of ores or the ash content of coal and in cement-making for determining the level of various elements in the raw materials (called the raw feed) which must be gauged for the manufacture of cement. In particular the following publications can be cited EP-A-O 171 256, U.S. Pat. No. 362,939 and WO-84/04393. The method is based on the fact that a bombardment of materials by neutrons causes nuclear reactions with various elements, accompanied by the emission of gamma rays which have different energy levels according to their origin, thus permitting spectrometric analysis. These rays include the so-called capture rays, which are only emitted during the capture of neutrons by atoms thus forming new isotopes, and the so-called activation rays, which are emitted by the radio-active isotopes thus formed and with a decreasing intensity during the life of these isotopes.
Under laboratory conditions, various neutron sources may be used to activate relatively small samples, placed in appropriate containers. But for continuos or intermittent analysis of large quantities in bulk, the size and manoevrability of the source of neutrons limits the possibilities. For such applications on an industrial scale, the standard mathod consists of using a radio-active source to bombard materials passing before it. For example, EP-A-O 171 256 described an installation for neutron activation according to this method and comprising a vertical rectangular funnel into which the bulk materials are continuously fed so as to run from top to bottom at a regular speed, one or more radio-active sources of neutrons arranged on one side of the funnel and one or more detectors arranged opposite the sources on the other side of the funnel in such a way as to obtain the most uniform results possible for the whole of the section of the flow of material.
The fact that radionucleides are permanent sources causes difficulties in such an industrial application. The extensive shielding necessary to protect the human environment, and the risks of contamination by accidental tearing away of the radioactive material of the source, obliges the source to be placed outside the flow of material, irradiation of this being effected by only a fraction of the neutrons emitted by the radionuclides and allowed to flow out of a window in the shielding. This arrangement imposes stringent precautions for the protection of personnel in the vicinity of the installation. Furthermore the section of the funnel therefore the volume of material exposed to the source at a given moment is limited, imposing the need for sampling in the control of large flows of material, such as are found for example in the exploitation or the pre-homogenisation of coal, of iron ore or of cememt raw feed. Also, the gamma ray detectors are exposed to neutron bombardment which may cause damage or deterioration, and to parasitic gamma rays from the two side walls of the funnel which are also the site of nuclear reactions. In addition, to allow for the density of materials in the zone situated between the source of neutrons and the detector opposite. EP-A-O 171 256 shows that extra means must be employed, for example two gamma ray sources situated on either side of the neutron source, and two supplementary detectors situated opposite the gamma ray sources. This increases the complexity of the installation and the problems of protection against radioactivity. Until now these drawbacks have prevented a wide application of the method and of this type of installation.
To limit the problems of radioactive protection, GB-A-2 101 304, in an apparatus designed to sort lumps of gold ore, provides a neutron source in the form of a target carrying deuterium and situated between three vertical rotating funnels arranged in a triangle around the source so as to form three passages for the materials. The target is bombarded with ions by means of a high voltage accelerator situated at the side and linked to the target by a vacuum tube. The gamma rays are detected later on in another part of the apparatus where the pieces are examined individually for sorting. In this way the activated gamma rays are only measured once. In an another side the proposed arrangement of the accelerator and rotating funnels causes considerable constructional difficulties.
Another problem with using radioactive sources is that near the source the gamma rays from capture and activation are measured simultaneously, never individually so that identifying the origin of the rays is made more difficult, even preventing exact measurement of the amounts of certain elements under investigation. The fact that certain activated elements have a very short half-life, of the order of a few seconds, means that a comparison cannot be made with a measure of activation energy made at a certain distance and a certain time after the activation.